This invention relates generally to X-ray apparatuses and in particular to beam filter positioning devices and linear accelerators incorporating the same.
Linear accelerators are used in a variety of industries including in medical radiation therapy and imaging. A linear accelerator includes a treatment head that houses various components configured to produce, shape or monitor a treatment beam. For example, a target produces X-rays when it is impinged by energetic electrons. A photon flattening filter shapes X-rays to provide a uniform dose distribution across the X-ray field. An ion chamber monitors the energy, dose distribution, dose rate, or other parameters of a radiation beam. In an electron mode operation, an electron scattering foil scatters incident electrons to provide a broadened, uniform profile of a treatment beam. A field light system simulates a treatment field by illuminating e.g. an area on the surface of a patient's skin.
In conventional accelerators, exchangers are used to position electron scattering foils and photon flattening filters. Foil-filter exchangers allow switching back and forth between scattering foils and flattening filters for electron or photon mode operations. Fine precision adjustments of the foils and filters in exchangers are accomplished in the factory by manually adjusting and testing the foils and filters, which is a very time consuming process.
Conventional foil-filter exchangers do not include a target assembly or field light assembly. In conventional accelerators the targets are located in other areas of the treatment head e.g. inside the accelerator vacuum envelope. The design of target assemblies residing inside the vacuum envelop is complex due to added vacuum walls and interface considerations. Actuation of targets in vacuum is complicated. Any water leaks in target cooling systems would contaminate the vacuum envelope causing extended downtime.
A field light system includes a lamp and a mirror, and is used to facilitate patient placement for treatment by providing an intense light field that coincides with the radiation treatment field shaped by collimator jaws or other beam limiting devices. Because of space limitations and other considerations, it is unfeasible to place a lamp in the same location as the radiation source. In conventional accelerators the mirror is fixedly disposed along the beam centerline and is made of a thin film that is generally transparent to radiation or electron beams. Once being installed, the mirror and the lamp projector are manually adjusted in order to achieve the required coincidence with the X-ray field. The mirror located in the beam centerline causes scattering losses and beam contamination. The thin film materials are susceptible to degradation due to exposure to radiation, damage and optical distortion.